`toAscUnfoldable` and `unionWith` for `StrMap`

[paulyoung]

I currently use these functions with Map and would like to use StrMap instead.

When attempting to do so, I realized that they weren't implemented.

If anyone has any thoughts or suggestions on the implementation of these, I'm interested in hearing them.

[matthewleon]

toAscUnfoldable is there now: 2c42ac7

unionWith remains to be implemented